(1) Field of the Invention
The present invention relates to systems and methods for recovering signals of interest from a phase modulated signal and more particularly, to a digital receiver system for receiving a phase modulated signal from a fiber optic interferometer and recovering a measurand signal by sampling the phase modulated signal using quadrature time samples.
(2) Description of the Prior Art
Fiber optic interferometers are highly sensitive devices for the measurement of time-varying measurand fields or signals, such as acoustic pressure, vibration and magnetic fields. The acoustic pressure changes, vibrations or magnetic fields affect the light transmitting characteristics of the optical fibers used in the interferometer, producing a change in the phase of the light signals traveling through the optical fibers. A measurement of the change in phase of the optical signal transmitted through the optical fiber is representative of the changes in the environmental conditions or measurand field acting on the optical fiber.
Typically, fiber optic interferometers utilize two optical paths or fibers, and an optical source, such as a laser, which provides a light signal in the optical paths. The measurand signal modulates the phase of the light signal in one or both of the optical paths, and the light signal thereby acts as a carrier for the measurand signal. The phase modulated optical signal is produced by interfering the optical signals in both paths, and the components of the phase modulated signal include both the carrier signal and the measurand signal.
One type of interferometric sensor system employs a phase generated carrier concept using two quadrature carriers, for example, having the frequencies .omega..sub.c and 2.omega..sub.c or 2.omega..sub.c and 3.omega..sub.c. The carriers can be generated by directly modulating the optical source in the interferometer at the carrier frequency .omega..sub.c or with an external phase modulator that provides the carrier frequency .omega..sub.c after the light signal has been generated. When the optical signal having the carrier frequency of .omega..sub.c passes through the interferometer sensors and is modulated by the measurand, the resulting phase modulated signal includes quadrature carriers at harmonics or multiples of the carrier frequency (e.g., .omega..sub.c, 2.omega..sub.c, 3.omega..sub.c, 4.omega..sub.c, . . . ) Using the quadrature carriers in the demodulation process prevents fading of the interferometric signal and normalizes the detected signal.
Various demodulation techniques have been used to recover the signal of interest or measurand signal from the phase modulated optical signal output from a fiber optic interferometer or other systems utilizing quadrature carriers. According to one technique, receiver systems convert the phase modulated signal from analog to digital and then use a homodyne technique to demodulate and recover the measurand signals from the quadrature carriers. To reconstruct the phase modulated signal, previous systems used sampling rates that satisfy Nyquist's criteria, i.e., a minimum sampling rate of twice the carrier frequency .omega..sub.c. If the system uses quadrature carriers at .omega..sub.c and 2.omega..sub.c, the minimum sampling rate according to the previous systems must be 4.omega..sub.c. The high sampling rates required by these systems (i.e., at least four times the lowest carrier frequency) places great demands on the sampling circuitry and limits the sensor bandwidth and the number of channels.
The use of analog circuitry, such as analog multipliers, filters, integrators and differentiators, in conventional receiver systems to demultiplex and demodulate signals from an array of interferometers also has a number of limitations and disadvantages. The components forming the analog circuits must be gain and phase matched with the carrier signal during the demodulation process to optimize performance, which is costly and difficult to achieve. Using the conventional homodyne technique, for example, to recover the measurand signal requires multiplication of the quadrature carriers by a local oscillator of the proper frequency, phase and amplitude. Failure to adequately match the amplitude and phase results in harmonic distortion, which reduces the useful dynamic range of the interferometer. The output level of prior art demodulators using analog circuitry is also limited by the power supply. Furthermore, noisy analog circuitry (e.g., multipliers and filters) can increase the noise floor of the system.
Fiber optic interferometer systems also commonly use an array of sensors to detect the measurand field over a larger area. One application for an array of fiber optic interferometer sensors is to detect acoustic waves, for example, in an underwater environment. If an array of fiber optic interferometer sensors are used, multiple signals are multiplexed on one or more fibers, for example, by using time division multiplexing (TDM) or wavelength division multiplexing (WDM) techniques. The phase modulated signal output from the array of fiber optic interferometer sensors must be demultiplexed to obtain the multiple signals of interest. One example of a TDM sensor system is sampled with a pulsed optical interrogation signal.